7. RESOLVING THE DISCREPANCIES

Wandering through the halls of astronomy departments around the country
(or even reading preprints on
the astro-ph archive), one hears a lot of interesting statements about
the large-scale bulk flow of galaxies within 6000 km s-1:

"The Lauer-Postman result cannot be right; it does not agree with
observed bulk flow measurements at 6000 km s-1 ."

"The Lauer-Postman result cannot be right; it does not agree with the
fact that the IRAS dipole
appears to have converged by 6000 km s-1."

"The observed bulk flow at 6000 km s-1 from Mark III is
inconsistent with the predictions of the IRAS redshift survey."

"The Mark III and da Costa et al.
[9]
dipoles are inconsistent with one another at 6000 km s-1 ."

Clearly, much of the current controversy centers around the bulk flow at
6000 km s-1. Stéphane
Courteau, Marc Postman, Dave Schlegel, Jeff Willick, and I have started
a full-sky Tully-Fisher survey of
galaxies specifically designed to nail down the bulk flow within a shell
centered at 6000 km s-1. We have
selected 297 Sb-Sc galaxies with 4500 < cz < 7000 km
s-1 with appropriate inclinations and without
morphological peculiarities, from the magnitude-limited full-sky
redshift survey sample of
[42]
(we decided
against using IRAS selection, given the large Tully-Fisher
scatter observed for IRAS galaxies in
[43]). The
sky distribution of this sample is shown in
Fig. 4b. For each galaxy, we
measure the rotation curve using a long slit for the
H line, and are doing
photometry in the Vand I bands. We have been
granted observing time at Kitt Peak and Cerro Tololo for this survey,
and we hope to finish in one year. Our estimate is that we will be able
to measure the bulk flow of this shell with an
error of 70 km s-1 , with an error ellipsoid that will be
close to isotropic. We believe that this
survey should resolve much of the controversy that is currently swirling
around this very hot topic.

I would like to acknowledge my collaborators in the various projects I
discuss here: the IRAS 1.2 Jy and
ORS redshift surveys (Marc Davis, Alan Dressler, Karl Fisher, John
Huchra, Ofer Lahav, Basílio Santiago,
and Amos Yahil), the POTENT/Mark III analysis (David Burstein,
Stéphane Courteau, Avishai Dekel,
Sandy Faber, and Jeff Willick), and the two bulk flow projects described
above (Stéphane Courteau, Tod
Lauer, Marc Postman, David Schlegel, and Jeff Willick). I acknowledge
the support of a Fellowship from the Alfred P. Sloan Foundation.